Modeling of critical lead loads on natural ecosystems of Belarus

Geography and Natural Resources 31 (2010) 283–290

Modeling of critical lead loads on natural ecosystems of Belarus S. V. Kakareka * and S. V. Salivonchik Institute of Nature Management NAS of Belarus, Minsk Received 30 March 2009

Abstract We give an outline of the model of behavior of heavy metals in terrestrial ecosystems and present the results derived from implementing it in estimating critical lead loads on natural ecosystems of Belarus. We provide the main methodological approaches, the algorithm, the composition of input information, the soil-geochemical, biogeocenotic, hydrological-climatic and other parameters used in the study as well as starting cartographic information. We provide the results of calculations in terms of the model as well as the maps of critical lead loads on the main types of natural terrestrial ecosystems of Belarus according to the effect-oriented and preservation approaches and demonstrate some cases where the existing levels exceed the calculated load standards. Keywords: heavy metals, lead, natural ecosystems, modeling, critical load.

Formulation of the problem The concept of critical loads evolved from the necessity of assessing the hazard of existing pollutants fallout deposition levels. This concept constitutes the core of the strategy formulated in the UNECE 1979 Convention on Long-Range Transboundary Air Pollution intended for a minimization of the consequences of the transboundary transport of chemical substances [1, 2]. The goal of establishing critical load standards is to determine the fallout deposition levels at which the pollutants starts to have a negative influence on the environment. Within the framework of the aforementioned Convention, the critical load on ecosystems is defined as a quantitative evaluation of the deposition level for one or several pollutants below which, according to data available to date, there is no detectable considerable adverse effect on specific sensitive elements of environment [3]. The notion “critical load” rests methodologically on the assumption that there exists a definite, relatively safe, level of noxious substance penetrating the components of the environment which the ecosystem is capable of utilizing or neutralizing without doing irreparable damage to its flora and fauna.

* Corresponding author. E-mail addresses: [email protected] (S. V. Kakareka), [email protected] (S. V. Salivonchyk).

Definition of critical load standards is built upon identifying, in one or several components of the ecosystem, a threshold level of pollutant concentration, an excess of which leads to irreversible changes in biogeochemical structure, biodiversity and productivity of ecosystems [4]. Calculations and mapping of critical loads are essential to an optimization of the air pollution control strategy, and to assessment of the efficiency and adequacy of the air protection measures undertaken with respect to pollutants. Originally, the critical load concept was applied to oxidizing and eutrophizing compounds; subsequently, it began to be extensively used also in regard to other pollutants. Further impetus to regional modeling of loads of heavy metals was provided by the signing of the 1998 Aarhus Protocol on Heavy Metals to the UNECE 1979 Convention on LongRange Transboundary Air Pollution [2]. A number of methods of evaluating critical loads for heavy metals have been developed to date, which differ in approaches, detail of investigation, and in receptor components. Among them are the dynamical and equilibrium groups of methods. The dynamical methods imply calculating the time prior to the attainment of maximum allowable concentrations (MAC) of a pollutant in the ecosystem or in its component. The equilibrium methods are built upon the mass balance equation for the metal in the soil system. The use of the balance method allows for a number of assumptions. It is assumed that the concentration of the metal in the soil has reached an equilibrium, the soil properties and the relationship between the forms in which the metal is in the

Copyright © 2010 IG SB, Siberian Branch of RAS. Published by Elsevier B.V. All rights reserved doi:10.1016/j.gnr.2010.09.015

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soil (adsorbed, associated with dissolved organic carbon or contained in the soil solution) are constant, and the horizontal (intersystem) flows are absent. When evaluating a critical load, it is customary to recognize two approaches. The effect-oriented approach (relying on effects) involves calculating the maximum possible flow of the metal into the ecosystem where the element concentration in the soil solution reaches a critical level. In the case of sustaining approach, a critical load is evaluated as the flow that acts to sustain the existing level of pollutant concentration in the soil solution [3]. The latter approach is a more environmentallyfriendly alternative; however, its implementation is confined to ecosystems where the existing content of matter in the soil solution does not exceed MAC. Because of their highly ready availability, the equilibrium methods have evolved dramatically and enjoy wide application. They can be categorized into two groups according to their detailedness: simplified methods that neglect flows of heavy metals of secondary importance and are based on a straightforward description of adsorption/ complexation processes as well as detailed methods that take into account all processes (flows) and are based on a more detailed consideration of such processes in the soil system. In addition to the computational method, of significant importance in the procedure of determining a critical load is the selection of the receptor (the sensitive component of the ecosystem) and ecological criteria (critical concentrations of heavy metals in the receptor). When considering the loads of heavy metals, attention is usually focused on the ecotoxicological risks to humans, soil microbes, higher plants, and to the soil and terrestrial fauna. Critical concentrations for each particular receptor are established according to ecotoxicological risks being evaluated [4]. The goal of this study is to develop the behavior model for lead in natural ecosystems as well as to perform, in terms of the model, an evaluation and mapping of critical loads of this metal on natural terrestrial ecosystems of Belarus. Objects and method of investigation The model algorithm. A simplified equilibrium method was used in evaluating critical loads for lead on natural ecosystems of Belarus [5]. The Coordination Centre for Effects (CCE) within the framework of the UNECE Convention on long-Range Transboundary Air Pollution at the National Institute of Health and the Environment (RIVM (Rijksinstituut voor Volksgezondheid en Milieuhygiëne), Bilthoven, the Netherlands) recommended it as the main procedure of evaluating critical loads for heavy metals; this procedure has gained widespread acceptance by various countries in regional evaluations [6]. Calculations were carried out concurrently with the use of the effect-oriented and sustaining approaches. The upper soil horizon (0–10 cm) was used as the receptor, and lead concentration in the soil solution was chosen as the ecological criterion.

To calculate a critical load for lead fPbtl(crit) with the effectoriented approach used the equations suggested in [5]. The adsorption of the metal by the wood increment fpbgu , and weathering of lead from the soil-forming earth materials fMwe were determined by the formulas used in [3]. The value of infiltration water flow in the upper soil horizon was calculated according to the equation: fle = P – k(frEi · P + frEse · P) – frru · Et , where P is the layer of atmospheric precipitation, m/year, k is the coefficient taking into account the difference in evaporation due to latitudinal variation of the amount of solar radiation, frEi is the coefficient of precipitation interception by vegetation, frEse is the coefficient of soil evaporation, frru is the share of the root absorption corresponding to the upper (0–10 cm) soil horizon, in fractions of unity, and Et is transpiration, m/year. The current metal content in the soil solution [Pb]tot,ss was calculated, based on concentrations of total forms of lead in the upper soil horizon (0–10 cm), for which purpose a nonlinear transform function was used: [Pb]tot,ss = ((ctPbs /Kf ) · Rssn)1/n , where ctPbs is total lead content in the upper soil horizon, mg/kg, Kf is the Freundlich absorption coefficient for lead, mol(1-n) · m3·n · kg-1 (with n being the dimensionless Freundlich constant), and Rss is the relationship between total content of free (not involved in complex compounds) lead in the soil solution. Because of scarcity of data for Belarus, the equations of nonlinear equilibrium distribution between metal phases in the soil system from [5] were used to obtain the original empirical dependencies: initially, the known total content of lead (Pbs ) was used to determine the concentrations of its mobile forms (Pbre ), whereupon the content of dissolved forms of the metal in the soil solutions was calculated from the inferred concentrations. The computational model. For calculating critical loads of heavy metals on ecosystems the “Metals” module was developed in the DBMS ACCESS 2000 environment. The model includes the database containing quantitative parameters of the objects under study, the computational unit, and the interface. Objects for computations can be represented by ecosystems of three hierarchical levels: regional, subregional, and local. On the regional level, the parameters of the objects are specified according to their typological belonging. Territorial differences of the parameters are additionally taken into account on the other two levels. For the purposes of this study, computations were done on the regional level. The computational unit of the “metals” model is a system of interconnected and sequentially arranged requests. The final request fMtl(crit) automatically activates and updates intermediate requests in which the flows of substance and metals in the ecosystem are evaluated: the infiltration water

S.V. Kakareka and S.V. Salivonchik / Geography and Natural Resources 31 (2010) 283–290

flow (request fle ), the metal bioabsorption (fMgu ), weathering of the metal from soil-forming earth materials (fMwe ), critical washout of the metal from the upper soil horizon (fMle(crit)), and the flow of existing washout of the metal from the upper soil horizon (request fMle). The final result is computed concurrently in terms of the sustaining and effect-oriented approaches. The digital cartographic base. For modeling the loads of lead on natural ecosystems, digital maps of soils and vegetation on the territory of Belarus were generated. The maps of vegetation and soils of Belarus we used as the cartographic base for them (sc 1:2 500 000) [7]. The scanning of the source maps, the transformation of the scanned images, and georeferencing and registration of separate sheets were carried out. ArcView GIS version 3.1 was used in the vectorization of the scanned images, the encoding of compartments in accordance with the classifiers of the source maps, and the determination of the connections of the compartments with the databases and the software package for computing critical loads. The classifiers of the electronic maps were constructed on the basis of the classifiers of the source thematic maps. The compartments of the vegetation map were used as the unit for computation and mapping of the loads. The types of soils were determined using the soil map for each study area. Published data [8, 9], material from forest management organizations (summary tables for th distribution of species according to the types of habitats), and indication tables used in interpretation procedures [10] were used in verifying the correspondence of the types of vegetation and soils, and their optimal, possible and intolerable combinations, and relevant updates were made. Parameterization. The parameterization procedure process of the model for soil-forming earth materials

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determined the weathering rates of the basic cations (BCwe, mol·ha-1·year-1·m-1), and the content of basic cations (CaO, MgO, K2O, and Na2O, %) and of total forms of lead (ctPbp, mg/kg). For soils the mean contents of organic matter (OM, %), clay (clay, %), total forms of lead (ctPbs, mg/kg), and the acidity index рН characteristic for the upper soil horizon (0–10 cm) as well as the coefficient of evaporation from the soil surface, i.e. the contribution from precipitation (frEse), were determined. For vegetation the procedure revealed the mean annual increment (Fgu , kg/m2 per year), the lead content in the increment (ctPbgu , mg/kg), the share of the root absorption from the 0–10 cm soil horizon (frru , in fractions of unity), transpiration (Et , m/year), and the coefficient of intercepted evaporation, i.e. the share of precipitation intercepted by vegetation (frEi ). The weathering rate of the basic cations from the soilforming earth materials was estimated following the technique suggested in [11], on the basis of information on the class of soil-forming earth material, and its texture. The total content of lead in the soil-forming material was determined by using data reported in [12–14], and the content of the basic cations was quantified according to data from [15–17] (Table 1). The soil acidity, and the content of clay (< 0.002 mm) and organic matter (losses due to calcination) were determined from data provided in [15–19] (Table 2). Note that in Belarus, a convention uses the determination of physical clay content by the Kachinsky method, < 0.01 mm. The necessary fraction (< 0.002 mm) for different soils is arbitrarily taken to equal the sum of silt content according to Kachinsky (< 0.001 mm), and 1/5 of the share of the soil fraction 0.001–0.005 mm. The content of the total forms of lead in soils was determined according to data reported in [12, 13, 16, 17, 19, 20] (see Table 2). Table 1

Parameters of soil-forming earth materials of Belarus as used in calculating the values of critical loads for lead Total content

Soil-forming earth materials

Weathering rate of basic cations, mol(1-n)· m3·n·kg-1

Pb, mg/kg

СаО

MgO

Lacustrine-glacial clays and loams Loesses and loessal loams Morainic clays and loams Aquatic-glacial loams Morainic and lacustrine-glacial sandy loams Aquatic-glacial and ancient alluvial sandy loams Morainic and lacustrine-glacial sands Aquatic-glacial sands Ancient alluvium Contemporary alluvium

2750 1250 1750 1750 1250 750 250 250 250 250

13 15 13 8 6 6 6 4 4 4

5.86 2.20 2.73 1.85 0.92 0.80 0.70 0.97 0.60 0.74

5.85 1.10 1.74 1.10 0.65 0.70 1.00 0.45 0.50 0.52

K 2O

Na2O

2.24 2.10 2.00 1.40 1.80 1.80 1.20 0.75 0.80 1.86

0.97 0.66 0.97 0.50 0.70 0.70 0.03 0.03 0.90 0.64

%

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Table 2

Parameters of soils of Belarus (0–10 cm horizon) as used in calculating the values of critical loads for lead рН (КCl)

Clay content, %

Losses due to calcination, %

Total lead content, mg/kg

Soddy-podzolic clay and loamy

4.4–6.4

11–23.5

3.9–5.2

15.0

Soddy-podzolic sandy-loamy

4.1–5.0

9.0–12.5

4.3–9.7

10.0

Soddy-podzolic sandy

3.0–4.6

4.0–6.5

1.6–5.0

6.0–8.0

Soddy-podzolic loessal

4.6–5.5

12.0–13.0

4.0–7.3

17.0

Podzolic

3.0

5.0

25.0

5.0

Soddy-calcareous

7.0

19.5

4.5

18.0

3.4–5.0

–

79.0–93.0

5.8

4.4

1.0

3.6

11.0

Soils

Peaty-boggy Alluvial soddy

The coefficient of evaporation from th soil surface was calculated as the ratio of the amount of soil evaporation to the amount of atmospheric precipitation on the basis of data from [21, 22] to be 0.22, 0.20, 0.18, 0.10 and 0.22 for clay, loamy, sandy-loamy, sandy and peaty-boggy soils, respectively. The annual increment for different types of forest in Belarus was determined using data from Goslesfond, growth rate tables, model calculations [23], and published information [24–27]. The content of lead in it was calculated according to the fraction of total mass of wood, bark, roots and needles (for conifers) involved in the formation of the increment, and metal concentration in this vegetation fractions. The absorption of lead from the upper soil horizon (0–10 cm) by plant roots was determined from data on the vertical distribution of the root mass [28]. For forest phytocenoses,

a calculation was performed from the fraction of total mass of the roots of the main forest-forming species in the upper soil horizon. The fraction equal to 75% was adopted for all open systems. Transpiration of vegetation was inferred from transpiration coefficients, and from annual production of vegetable matter (Table 3). The following values of the coefficients of precipitation interception by vegetation were used: 0.29, 0.22, 0.2 and 0.1 for spruce, pine and larch forests and for herbaceous vegetation, respectively [29, 30]. The coefficient taking into account the latitudinal variability of the value of total evaporation, and also the distribution of the annual amount of precipitation on the territory of Belarus were obtained according to [7]. Table 3

Vegetation parameters as used in calculating the values of critical loads for lead Annual increment, g/m2 per year

Content in increment, mg/ kg of dissolved matter

Share of root absorption from 0–10 cm soil horizon, fractions of unity

Transpiration, m/year

pine forests along waterless valleys

0.26–0.30

3.4–3.7

0.63

0.112–0.192

pine forests on transitional and high bogs

0.13–0.22

3.0–3.3

0.66

0.033–0.054

0.31

6.4

0.83

0.136

broad-leaved/coniferous forests

0.28–0.36

4.1–4.4

0.50–0.58

0.163–0.168

oak forests

0.21–0.33

1.4

0.35

0.180–0.258

black-alder and grey-alder forests

0.20–0.23

3.3–4.4

0.30–0.35

0.147–0.202

aspen forests

0.36

3.3

0.34

0.214

derivative birch forests

0.23

4.4

0.55

0.179

0.15–0.20

4.4

0.55

0.099–0.102

0

0

0.75

0.180–0.360

Groups of vegetation

Forests:

spruce forests

downy-birch forests on transitional and low-level bogs bog and meadow vegetation

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Research results and discussion The lead weathering flow from the upper soil horizon (0–10 cm) varies from 0 to 071 g/ha per year, averaging 0.2 g/ha per year for the territory of natural lands of Belarus. Maximum weathering values were obtained for soils occurring on heavy clays and loams as well as on loesses and loessal loams; very low values were obtained for soils forming on alluvial and aquatic-glacial sands, and zero values corresponded to organogenic soils. The lead bioabsorption flux for forest ecosystems of Belarus varies across the territory from 1.3 to 16.7 g/ha per year, averaging 5.2 g/ha per year. The largest bioabsorption value was obtained for spruce forests, and a somewhat smaller value was inferred for pine and broad-leaved/spruce forests. The least amount of metal is absorbed by the increment in biomass of broad-leaved forests. The calculated background concentrations of dissolved forms of lead in soils of natural lands range from 0.19 to 12.13 mg/m3, averaging (or weighted-mean values for the areas) 3.7 mg/m3. The lowest concentrations are recorded in soil solutions of soddy-calcareous, organogenic, soddypodzolic clay and soddy-podzolic sandy soils. The infiltration flow washing out the upper soil horizon, evaluated for a year with average water abundance, averages for natural ecosystems 0.27 m/year (the weighted-mean value), varying from 0.01 to 0.45 m/year. The least infiltration is observed in forestless ecosystems on peaty-boggy soils, and the largest infiltration is recorded in forest ecosystems on sandy soils. The resulting value of critical washout of lead in the same

ecosystems ranges from 0.8 to 36 g/ha per year, averaging 21.6 g/ha per year, and represents the largest (in magnitude) flow in the balance equation which determines to a large measure the value of critical load. A calculation of permissible loads for lead in natural ecosystems showed that they vary over an extremely wide range: from 0.4 to 43.7 g/ha per year when the sustaining approach is used, and from 8 to 41.7 g/ha per year when the effect-oriented approach is used (Fig. 1). The values of threshold loads, calculated on the basis of the sustaining approach, are substantially lower for almost all ecosystems than those obtained by using the effectoriented approach; they differ by a factor of 1.5–2, on the average. In general, the sustaining approach ensures a higher protectedness of ecosystems; however, the results have exceedingly high variability and very low values for individual ecosystems, which is due to the unadaptedness of the functions describing the transition of heavy metals from the absorbing soil complex to the soil solution, to the conditions of Belarus. This reduces the practical merits of the particular method. Table 4 provides the resulting values of critical loads for ecosystems of different types, with a different degree of their protectedness. An examination of the data reveals that the forest ecosystems of Belarus are much more resistant to the atmospheric load of lead when compared with meadow and bog ecosystems. The indices of critical loads on this territory exceed by a factor of 1.3–1.4 the average indices obtained for all natural ecosystems. Amongst the forest formations, the largest values of

Fig. 1. Levels of critical loads for load in natural terrestrial ecosystems of Belarus, g/ha per year. Approach: а – sustaining, b – effect-oriented.

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critical load for lead were revealed for spruce forests (Table 5), which is largely due to the high level of bioabsorption. According to our calculations, the mean values of critical loads of lead for these ecosystems are by a factor of 1.3–1.5 (depending on the particular approach) higher than those for the forests of Belarus as a whole. Values of critical loads of lead vary also with soil cover. The most resistant and the most sensitive to lead pollution are the ecosystems forming on light sandy soils and on peaty-boggy soils, respectively (Table 6). On the whole, it

should be remarked that under the conditions of identical loads of heavy metals, the bog and meadow ecosystems are jeopardized the most; this is especially true where these ecosystems occur on organogenic soils. A comparison of critical load levels for load with those obtained following a similar technique for other countries [31] showed that our data fit in the range of variation of the values calculated for natural ecosystems for countries in Europe. For most European countries the critical loads for this metal for natural ecosystems vary from 0 to 50 g/ha per year. Table 4

Values of critical loads for lead for different types of natural ecosystems of Belarus with a different degree of protectedness, g/ha per year Share of protected ecosystems, % Type of ecosystems

5

25

50

75

95

1

2

1

2

1

2

1

2

1

2

Forest ecosystems

35.86

29.88

31.71

24.01

28.20

14.53

25.47

8.94

21.78

4.01

Meadow and bog (open) ecosystems

21.81

20.81

18.40

16.33

16.48

6.0

15.03

1.56

11.66

0.58

Note. 1 – effect-oriented approach, 2 – sustaining approach. Table 5

Mean values ( x ) and standard deviations (sx ) of calculated critical loads for lead for different forests of Belarus, g/ha per year Approach sustaining

Forests

effect-oriented

x

sx

x

sx

Pine forests

17.2

8.1

29.3

4.2

Spruce forests

24.7

5.7

36.2

1.9

Broad-leaved/coniferous forests

15.1

8.3

25.3

3.2

Broad-leaved forests

14.0

8.1

26.1

2.9

Small-leaved derivative forests

16.3

7.7

27.7

2.5

Small-leaved native forests on bogs

14.5

10.2

30.4

3.6 Table 6

Values of critical loads for lead in natural ecosystems of Belarus depending on the type of soils and the degree of protectedness of ecosystems, g/ha per year Degree of protectedness, %

For all types of natural ecosystems

Mineral soils loamy soils

loamy sand soils

sandy soils

Organogenic soils

1

2

1

2

1

2

1

2

1

2

5

35.06

29.00

34.00

30.12

36.46

26.36

35.60

30.33

31.80

8.92

25

30.30

21.77

27.26

19.50

31.39

16.08

32.73

27.37

27.92

6.45

50

26.43

13.59

25.11

14.50

28.15

13.52

29.55

24.81

25.27

5.32

75

21.07

6.45

19.26

6.72

24.13

11.84

25.97

21.21

16.21

1.68

95

14.50

1.34

16.00

2.23

14.79

6.00

19.89

6.86

11.70

0.90

Note. 1 – effect-oriented approach, 2 – sustaining approach.

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Noteworthy are three countries (Cyprus, France, and Sweden) for which lower values of critical loads for lead than in the case of Belarus were obtained. Another group of countries (Belgium, Germany, and Great Britain) has a wider range of values of critical weathering and loads for lead, with the average values being slightly higher when compared with Belarus. The closest estimates were obtained for Poland, Bulgaria, and the Netherlands. Excesses of critical loads for lead for the territory of Belarus were inferred by correlation with actual atmospheric fallout deposition levels for this pollutant through the use of data from Meteorological Synthesizing Centre-East (Moscow, Russia) obtained via model calculations within the Co-operative Programme for Monitoring and Evaluation of Long-Range Transmission of Air Pollutants in Europe (EMEP). According to these data, the specific loads for lead in Belarus vary from 4.2 to 21.5 g/ha per year against the average value 6.7 g/ha per year. In calculations based on using the effect-oriented approach, excesses of critical load levels were observed only for a very small part of the territory of Belarus, 0.27%. They are characteristic primarily for open ecosystems, and almost all of them occur in areas with very high lead fallout deposition levels (in excess of 16 g/ha per year) (Fig. 2, а). The excesses of permissible fallout deposition levels as obtained by using the sustaining approach are recorded in 14% of the area of all natural lands, averaging 3.7 g/ha per year (see Fig. 2, b). These excesses were revealed mainly in open ecosystems in areas where the lead fallout deposition levels are more than twice as large as the country-average levels.

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The data obtained in this study lead us to suggest that, with the existing lead fallout deposition levels persisting, about a seventh part of all natural systems will show an increase in pollutant concentration in the soil, and this will be taking place largely in ecosystems. However, an excess of dissolved lead concentration over the permissible level of 8 mg/m3 must not be recorded in more than 1% of the areas of natural lands. Conclusion The values of critical loads, calculated by the equilibrium simplified method for the natural ecosystems of Belarus, are comparable with the indices of loads obtained for other countries. They vary considerably with the type of ecosystems, and with the approach used. The indices, obtained by using the effect-oriented approach, are considerably larger when compared with the sustaining approach: the 95 %-degree of protectedness of natural ecosystems in accordance with the former and latter approaches is provided by the lead fallout flux not exceeding 14.5 and 1.34 g/ha per year, respectively. Our calculations intimate that the most resistant to fallouts are the forest ecosystems (this applies primarily for spruce forests) as well as those occurring on sandy soils with a high level of infiltration. Open ecosystems are more sensitive to lead pollution; with the current lead fallout deposition levels persisting, there will be taking place an increase in concentration of this metal in the soil solutions throughout a major portion of these ecosystems.

Fig. 2. Distribution of relationships between lead fallout deposition levels and values of critical load for natural ecosystems of Belarus. Approach: а – sustaining, b – effect-oriented. Lead fallout depositions: 1 – do not exceed critical loads, 2 – exceed critical loads.

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